Over the years there has been much research and development effort focused on the potential for developing a commercially viable Homogenous Charge Compression Ignition (HCCI) internal combustion engine for motor vehicles. A primary interest in developing a commercially viable HCCI engine for motor vehicles is that such an engine would theoretically have comparable efficiency to a conventional diesel engine (i.e. greater than a conventional gasoline engine), but with near zero production of harmful NOx and PM emissions. As of today, development of a commercially viable HCCI engine has never been successful because of (i) the difficulty in effectively controlling the initiation of combustion in a multicylinder HCCI engine over the changing speed and load conditions that would be involved in normal operation of a motor vehicle, and (ii) the difficulty in controlling initiation of combustion in an HCCI engine at full load.
Unlike with conventional gasoline engines (with combustion triggered by spark ignition), or diesel engines (with combustion triggered by late cylinder fuel injection), the start of ignition in HCCI engines is very hard to predict when the various operative parameters for combustion are in flux (e.g. intake charge-air temperature, cylinder wall temperature, boost pressure, charge-air oxygen concentration, fuel quantity, etc), and (in the case of cylinder wall temperature) even vary cylinder to cylinder.
As a result, HCCI research engines are only now beginning to be able to operate in multicylinder conditions, generally at steady state conditions and partial load conditions (e.g. with a maximum of about 75% of full load), with some minimal capability for slowly transitioning HCCI operation from one steady state condition to another for a change in power demand from the engine. Such transient ability for HCCI engines is currently far too slow (on the order of ten to one hundred times too long) for a commercially acceptable response the for use in a conventional motor vehicle. For example, for commercially acceptable responsiveness in a conventional vehicle, the time allowed for an engine to adjust from low power output to a relatively high power output would be a fraction of a second, whereas (without a major breakthrough) the best current HCCI engines would take multiple seconds to successfully make such a significant upward power transition in HCCI mode.
In addition, HCCI engines currently preferably use some time (e.g. 20–30 seconds) after the engine is turned on to operate in a non-HCCI mode (e.g. in spark Ignition mode), until the various operative parameters for successful HCCI combustion stabilize to desired levels, before HCCI combustion is initiated. Alternatively, the engine may be made to start quickly in HCCI mode if the engine is already warm (e.g. coolant and oil temperature are above 50° C.), such as through non-HCCI operation or external warming.
For at least these reasons, HCCI engines are currently far from being considered a viable option as a power plant for conventional motor vehicles.
Given the above limitations of HCCI engines, use of an HCCI-engine in conjunction with a hybrid powertrain (e.g. internal combustion engine/electric or internal combustion engine/hydraulic) motor vehicle would at first seem to only exacerbate the complications and shortcomings of an HCCI engine. This is because virtually all hybrid powertrain methods of operation not only operate the engine through rapid transients, but also rely on frequent cycling off of the internal combustion engine (or individual cylinders thereof) in order to avoid fuel consumption when the vehicle may instead operate through the secondary (e.g. electric or hydraulic) power source. However, as discussed above, rapid transients and rapidly cycling of an HCCI engine off and on during motor vehicle use would not be conducive to use with present HCCI technology (e.g. because of the delays for the engine to be able to transition between engine operating states or into HCCI mode), and thus an HCCI engine would appear to be an illogical match for a potential hybrid vehicle.